Transparent windshields for various vehicles, such as cars, rail vehicles including trains, streetcars, and locomotives, snowmobiles, airplanes, helicopters and sea vessels, must be deiced or defrosted using available on-board power. Typically, deicing and defrosting are accomplished by blowing air heated by the vehicle's engine onto the windshield. However, especially since the engine is initially cold upon startup, deicing/defrosting takes a considerable amount of time.
To deice a windshield in less than thirty seconds, a high voltage (typically over 100V) and high power (typically greater than 3 kW) must be applied to an electrically heated windshield. Common 12V DC power sources, found in most commercial and passenger vehicles, are able to deliver up to 10 kW of power but only into extremely low resistance loads, such as 0.01 ohms. A conductive film windshield heater, to be sufficiently transparent, must have a resistance of over 1 ohm. Thus, traditional 12V power sources are unable to meet the requirements of a rapid windshield deicing system with a transparent windshield heater.
Previous attempts to increase on-board voltage have involved either disconnecting an alternator from a battery and increasing idle rotation speed (see, for instance, U.S. Pat. No. 4,862,055) or feeding a step-up transformer with non-rectified AC current from an alternator (see, for instance, U.S. Pat. No. 5,057,763). In both cases, output power was limited by the size of the alternator such that the voltage necessary for rapid windshield deicing could not be achieved without significant resizing of the alternator. Moreover, since an alternator generates low-frequency power, a step-up transformer of sufficient output power would be heavy and costly to manufacture. An unregulated alternator has been used to directly supply electrical heating power in U.S. Pat. No. 3,572,560.